Bill says he thinks he's adopted, and it turns out that he is. Given that he has brown hair, while his parents both have fair hair, it would be impossible for him to be their natural child. His sister, who is their child, has fair hair like them.
Martin is introduced as a stereotypical nerd, getting trounced during his debate with Bill. Later, he's revealed to be part of Ted's cool crowd and displays none of those nerdy mannerisms from then on. It's ultimately revealed that Society has been manipulating Bill for his whole life, so Martin's sandbagging of the debate was all part of it.
